Is there an easy way to force the game to only spawn enemies from the expansion's selection in a test room?

Choosing the golden/crystalline floor styles still spawns whatever the game feels like spawning and I would prefer not having to tweak the spawning rules of every single enemy in the base game.

Unfortunately, there's no legit way to do that. For my last room batch, I just deleted the relevant base enemy entity XML files, which produces a ton of errors on startup, but does the job. I can then just update via SVN to get those files back, but obviously you can't do that, so it becomes a little trickier. I think you're probably going to need to create a temporary enemy entity folder, minus the base game enemies, or something like that. You could set all those enemies to not seed, of course, but that's more work, yeah. We've had to do a lot of workaround style things to keep the expansion moving forward, which can be a bit inefficient, but at least it works, I guess. I could perhaps look into setting that up correctly in the engine myself, but having me looking at engine stuff is generally SUPER inefficient because of my very very basic coding knowledge.